Recently, in the optical communication field, there has been a demand for larger capacity and higher speed in information transmission. In order to meet this demand, various constituent technologies, such as WDM (Wavelength Division Multiplexing) have been studied. In such high-speed, large-capacity optical communication being performed, polarization mode dispersion (PMD), which was a problem conventionally, is noted as a significant parameter for imposing limitations on transmission characteristics of an optical signal. Then, studies are being carried out about a PMD compensator for compensating for the effects of PMD.
There are many propositions made as to the above-mentioned PMD compensator, which are disclosed in a Japanese Laid-Open Patent Publication Nos. H11-196046, 2000-507430 and 2000-031903.
According to the Japanese Laid-Open Patent Publication No. H11-196046, an optical signal transmitted on a transmission line is composed of two orthogonal polarization components called “Principal States of Polarization” (PSP). A polarization mode dispersion compensator disclosed in this publication includes a polarization controller for subjecting these two polarization components to polarization conversion into two orthogonal Eigen States of Polarization (ESP) of a DGD (Differential Group Delay) emulator, detecting means for detecting waveform distortion by polarization mode dispersion of a transmitted optical signal and a controlling device for controlling the operation of the polarization controller with a control signal from the detecting means.
A polarization mode dispersion compensator disclosed in the Japanese Laid-Open Patent Publication No. 2000-507430 is provided with a DGD emulator anterior to a receiver. A DGD of the DGD emulator is a fixed amount and is larger than polarization mode dispersion which occurs in an optical path including a transmission line between a transmitter and the DGD emulator and a polarization controller. The polarization mode dispersion compensator is configured to detect a DOP (Degree of Polarization) of light output from the DGD emulator and to control a polarization controller so that this DOP represents a maximum value. With such control, either PSP (Principal States of Polarization) of the optical path extending between the transmitter and the receiver and including the transmission line, the polarization controller and the DGD emulator is aligned with a SOP (State Of Polarization) of light transmitted from the transmitter.
A polarization mode dispersion compensator disclosed in the Japanese Laid-Open Patent Publication No. 2000-031903 is similar to that disclosed in the publication No. 2000-507430 in that a DOP is used as a control amount. However, the polarization mode dispersion compensator disclosed in the publication No. 2000-031903 utilizes as DOP detecting means a polarization analyzer as well as a polarization controller and a polarizer as described in the publication No. 2000-507430.
All the above-mentioned related arts are effective only at compensating for first-order polarization mode dispersion. However, when the polarization mode dispersion compensator is actually applied to the transmission line, there exists another problem of compensating for second-order polarization mode dispersion. Means for compensating for this second-order polarization mode dispersion is disclosed for example in OFC2002, WI4, Technical Digest p. 236 (hereinafter referred to as “cited reference 1”).
Specifically, a polarization mode dispersion compensator disclosed in the cited reference 1 is configured to be such a polarization mode dispersion compensator as disclosed in the publication No. 2000-507430 being provided with a polarization controller and a polarizer at the latter stage thereof. According to the cited reference 1, an output from the polarizer arranged in the latter stage is maximized to align output polarization from the polarization mode dispersion compensator to linear polarization and remove polarization components other than linear polarization components. With this configuration, a depolarize component, which is one of components caused by second-order polarization mode dispersion, can be removed.
However, in each polarization mode dispersion compensator disclosed in the above-mentioned publications H11-196046, 2000-507430 and 2000-031903, the detecting means provided therein has a complex configuration, which presents a problem of extremely high cost.
Specifically, in the publication No. H11-196046, the intensity of intensity-modulated signal light is directly measured, subjected to photoelectric conversion into an electric signal and then, a part of the intensity-modulated frequency thereof is cut off by an electric filter so as to use its intensity as a control amount. However, this configuration requires a modulation component of very-high frequency component to be detected and subjected to circuit processing. Such electric circuitry is generally complex and very expensive.
In the publication No. 2000-507430, the DOP detecting means has complex configuration including a polarization controller, a polarizer, a power monitor anterior to the polarizer, a power monitor posterior to the polarizer, and an operating circuit for calculating a DOP by comparing power obtained by the power monitor anterior to the polarizer with power obtained by the power monitor posterior to the polarizer. Such DOP calculation is time-consuming, which results in further time-consuming control.
In the publication No. 2000-031903, the polarization analyzer itself is very expensive and time-consuming in DOP calculating.
Then, if the second-order polarization mode dispersion compensating means described in the cited reference 1 is used in each of these complicatedly configured polarization mode dispersion compensators, the configuration of the polarization mode dispersion compensator would be more complicated and inevitably expensive.
On the other hand, a polarization mode dispersion compensator as mentioned above in the related art functions effectively only for a signal wavelength. For this reason, when being applied to a wavelength division multiplexing communication system, such a polarization mode dispersion compensator has to be arranged for each channel, and thus, multiple polarization mode dispersion compensators are to be arranged in a lump. Accordingly, the polarization mode dispersion compensator is desired to be simple in configuration and low in cost for the purpose of widespreading and practical application of such polarization mode dispersion compensators.
Further, the polarization mode dispersion (PMD) is a main factor for deterioration in an optical communication system at 40 Gbps and above. Depending on a PMD amount of the transmission line, it may become a problem also in a 10-Gbps system which utilizes an older fiber (Cited reference [1]: F. Bruyere, Optical fiber Tech., 2, pp. 269-280, 1996). For this reason, many studies have been carried out about PMD compensation (Cited reference [2]: H. Ooi et al., OFC99, WE5-1, P. 86, 1999; Cited reference [3]: T. Takahashi et al., Electron. Lett., 30(4), pp. 348-349, 1994; Cited reference [4]: F. Roy et al., OFC′ 99. TuS4-1, P. 275, 1999; Cited reference [5]: C. Francia et al., Photon. Technol. Lett., 10(12), p. 1739, 1998; Cited reference [6]: J. Poirrier et al., OFC2002, W14, P. 236, 2002). These arts are directed to a single wavelength and a single channel, and in order to be actually applied to a multi-wavelength, multi-channel system by wavelength division multiplexing, they must be simple in configuration and inexpensive.
Accordingly, the present invention was carried out in order to solve the above-mentioned conventional problems. It is an object of the present invention to provide a polarization mode dispersion compensator which has detecting means of simple configuration, is allowed to compensate first-order polarization mode dispersion and second-order polarization mode dispersion and is low in cost, a polarization mode dispersion compensating method thereof and its application to an optical communication system.